Synthesis of Nitrocellulose

Introduction

Nitrocellulose is a highly flammable polymer made from Cellulose. Burning small quantities of it makes a very entertaining flash. It is also relatively safe to burn, as you will see, making it a great compound for demonstrations or just to amaze someone. It is also known as Flash Paper, Flash Cotton or Guncotton.

Follow this link to see the remarkable burning of Nitrocellulose!

Reagents

• Source of Cellulose – cotton, 100% cotton paper, 100% cotton clothes, etc;
• Nitric Acid 60% (greater concentration can also be used but quantities must be adjusted – check Theory section);
• Sulfuric Acid 98%;
• Sodium Bicarbonate (Sodium Carbonate can also be used, as well as the Potassium variants);

Dangers

Parts of this experiment will involve the production of toxic gases. Thus, the experiment must be carried outside or in a Fume Hood.

Very corrosive acids are going to be used. A mixture of those very corrosive acids is also going to be handled. Goggles and gloves must be used.

• 98% Sulfuric Acid is extremely corrosive. Besides, it is able to dehydrate organic matter, turning it into a pile of carbon. Your hands and eyes are organic matter, so be extremely careful and use gloves and goggles.
• 60% Nitric Acid is also extremely corrosive and releases toxic vapors. It must be used outside or in a Fume Hood. Wear gloves and goggles.

Procedure

Beforehand, be sure to have ice standing by, ready to be used. It is also recommended chilling the Sulfuric Acid and the Nitric Acid before using them.

So, start by setting up an Ice Bath and securing a beaker in it.

Note: I’m not using a beaker. Instead, I’m using a regular glass jar. Honestly, I’m not using a beaker because the only tall version beaker I had met his end a few days ago. And you want a vessel that is tall and straight so there is more cooling area. This goes to show that this part can be done without using Borosilicate Glass, if the temperature is controlled carefully.

Then, add 30mL of 60% Nitric Acid into the beaker. Now add, slowly, 80mL of 98% Sulfuric Acid. If you’re using 70% Nitric Acid, maintain the 30mL of Nitric Acid but only use 70mL of Sulfuric Acid. Add a thermometer and monitor the temperature.

Note: it is recommended covering the top of the beaker with cellophane (parafilm is better) to avoid Nitric Acid vapors coming out.

The temperature of my solution rose to about 55ºC.

While the solution chills in the ice bath, prepare your cellulose source. Weight 5 (you can use 6 if you’re using 70% Nitric Acid) grams of it and set aside.

When the solution reaches 20ºC, add the 5g (or 6g as explained) of cellulose into it. Press down on it (using a stir rod) several times. This allows the cellulose to absorb the solution.

Allow everything to react for about an hour. Every 15 minutes, press down on the cellulose several times.

After an hour, dump the solution into a beaker filled with at least half a liter of water. This effectively dilutes the acid, rendering the solution “safe”. Be sure to press the Nitrocellulose (yes, now you have Nitrocellulose) to remove most of the acidic solution out of it.

Add water to the Nitrocellulose, press it, and drain the water. Repeat this process three times.

Note: now we get to a very important part. We must neutralize the Nitrocellulose completely. When not properly neutralized, Nitrocellulose doesn’t burn as beautifully and leaves residues.

I have a particular method of neutralizing it. With a gloved hand, I remove the Nitrocellulose from the beaker and press it down. I do this in a sink. I open the tap and allow a flow of water to continually splash the Nitrocellulose. While this is happening I make a fist, pressing the Nitrocellulose. I repeat the fist part at least thirty times.

Prepare half a liter of a saturated solution of Sodium Bicarbonate (or the alternatives mentioned on the Reagents section). Add the Nitrocellulose to it and stir. Leave it there for 5 minutes.

Now repeat the wash you did before, in the sink. Gloves aren’t necessary anymore.

When you’re finished, press the Nitrocellulose in your hand to remove as much water as you can.

Here’s the undried product:

Note: The next part is the drying part, equally important. For a nice clean burn, the Nitrocellulose must be perfectly dried.

There are a lot of ways to dry it. For example, in the winter I dry mine next to my grandmother’s fireplace. Space heaters and oil heaters are also good. However, because none were available at the time, I dried mine in my hotplate. Be sure to do this in the minimum setting of your hotplate.

Here is some of it drying (I dry small pieces at a time):

So, how do you know it is dried?

When it dries, there is a pale color change, from a yellowish cotton to a very pale/almost white cotton. Here’s a picture (top is dried, bottom hasn’t dried yet):

To be sure, take a small sample and burn it. If it burns quickly, without residue, it is done. If it takes some time and leaves black residue, it hasn’t dried fully yet.

Note: Be careful with the temperature of your heat source. Too high and it will deflagrate the Nitrocellulose. For instance, my hotplate has 5 heat settings. If I leave dry Nitrocellulose on the “3” setting, it deflagrates in a matter of seconds.

And now you have a nice quantity of Nitrocellulose. Use it wisely. I placed a demonstration of it burning in the Demos section:

https://hobbychemistry.wordpress.com/2015/04/03/burning-nitrocellulose/

Theory

To better understand the actual Nitration process, please read my post about it:

https://hobbychemistry.wordpress.com/2015/04/01/nitrations/

So, what’s happening is the following reaction:

(C₆H₁₀O₅)_n(s) + 3NO₂⁺(aq) –> (C₆H₇(NO₂)₃O₅)_n(s) + 3H⁺(aq)

So, three moles of Nitronium ions, NO₂⁺, nitrate one mole of Cellulose, (C₆H₁₀O₅)_n, to form one mole of Nitrocellulose (C₆H₇(NO₂)₃O₅)_n. Also, one mole of Nitric Acid generates one mole of Nitronium ions, as seen on my post about Nitrations.

Molar Masses

Cellulose: 162,14 g/mol

Nitrocellulose: 297,15 g/mol (if fully nitrated)

Nitric Acid: 63,01 g/mol

If this were a straightforward reaction, 189,03g of Nitric Acid would fully nitrate 162,14g of Cellulose. So, 315,05g of 60% Nitric Acid (430,58mL) would fully nitrate 162,14g of Cellulose. Scaling it down by a factor of 10, we get 43mL of 60% Nitric Acid fully nitrating 16g of cellulose. But, as explained, this is a sensible equilibrium. Thus, an excess of Nitric Acid should be used. Scaling down, again, by a factor of 1,43, gives 30mL of 60% Nitric Acid to 11g of Cellulose. I chose to use about half of the theoretical Cellulose amount.

Sulfuric Acid’s role, as explained on the post about Nitrations, acts as a catalyst and dehydrating agent. The quantity one should use of it varies from source to source. I recommend the quantities given in the procedure.

Note: The quantity of Sulfuric Acid to be used diminishes when using 70% Nitric Acid because there is less water spoiling the equilibrium.

Finally, what is “fully” nitrated Cellulose?

Let’s take a look at the structure of Cellulose (image from http://upload.wikimedia.org/wikipedia/commons/0/07/Cellulose_Sessel.svg):

Now, let’s take a look at the structure of Nitrocellulose (image from http://upload.wikimedia.org/wikipedia/commons/8/83/Nitrocellulose-2D-skeletal.png):

The only difference is that the Hydrogen in the OH group gets replaced by the Nitro Group. Now, if the conditions aren’t right, some OH groups won’t be affected, leaving a less nitrated product. Fully Nitrated Cellulose is the one that gives the best “show”.

Final Notes

As usual, please comment and leave some feedback. Also, don’t forget to check the demonstration video of the Nitrocellulose made here.